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Whole-House Dehumidification

Scope

In “Warm-Humid” climates as defined by the 2009 IECC, install additional dehumidification equipment or an HVAC system equipped with controls to operate in dehumidification mode to maintain indoor relative humidity (RH) at or below 60 percent.

DOE Zero Energy Ready Home Notes

The U.S. Department of Energy (DOE) Zero Energy Ready Home National Program Requirements includes as a mandatory requirement (Exhibit 1, Item 6) that all homes meet the U.S. Environmental Protection Agency Indoor airPLUS Construction Specifications.

NOTE: Completion of the ENERGY STAR checklists now satisfies the following Indoor airPLUS requirements:

Properly size all heating and cooling equipment to accommodate design loads for each room as determined using ACCA Manual J, ASHRAE Handbooks, or equivalent software, as well as the pressure drop from all specified filters (HVAC-C Section 2).

A central HVAC system equipped with additional controls to operate in dehumidification mode.

Exception: Climate Zones 4-8, 3B, 3C and the portions of 3A and 2B above the white line as shown by 2009 IECC Figure 301.1.

Advisory: Although not required to meet this specification, independent dehumidification is recommended in Climate Zones 4A and 3A above the white line as shown in 2009 IECC Figure 301.1

Description

Air conditioning provides two key benefits that help enhance indoor comfort: cooling and dehumidification (removing sensible heat and latent heat). Removing sensible heat creates a cooler air temperature while removing latent heat removes humidity from the air. If the HVAC system’s coil temperature is below the dew point of the air (the temperature at which water vapor will condense into liquid water), the air conditioning system will simultaneously lower the air temperature and remove moisture. Together, sensible heat and latent heat constitute the cooling load on an HVAC system.

For maximum effectiveness, a well-designed HVAC system depends on an accurate room-by-room calculation of the heating and cooling loads. The system cooling load is calculated by summing the amount of sensible heat and latent heat that needs to be removed from the air. Important factors that contribute to sensible and latent heat calculations include outdoor climate, building orientation, building design, and other factors.

The optimal cooling system will be sized to meet its projected loads, resulting in better temperature and humidity control. Oversized cooling systems can cause short-cycling of equipment (i.e., short periods of “on” time when cooling is provided followed by long periods of “off” time when no cooling is provided). In homes with over-sized air conditioners, the thermostat quickly reaches the desired set point before moisture can condense on the coil and be removed from the home. In this situation the temperature in the home drops but the relative humidity climbs.

In addition to creating an uncomfortable space, indoor relative humidity consistently greater than 60 percent can promote mold growth and create an environment conducive to dust mites and other pests. In hot, humid climates, additional dehumidification may be necessary. Accurate load calculations based on the home’s envelope characteristics, climate zone, and specific orientation can help address these issues by aiding in the proper sizing and design of cooling and dehumidification systems.

In order to ensure proper sizing of heating and cooling systems, the HVAC industry uses “Manual J,” a calculation worksheet developed by the Air Conditioning Contractors of America (ACCA). The Manual J worksheet accounts for factors such as the local climate, orientation of the home and the thermal performance of the building enclosure including windows, roofs, walls and floors. After the completion of a Manual J load calculation, the HVAC designer then uses a procedure (Manual S) to properly select equipment capable of meeting both the sensible and the latent loads in the home. In hot-humid climates, the air conditioner alone may not be able to remove enough latent heat to keep relative humidity below 60 percent. In such cases, HVAC design must also consider any additional dehumidification capacity, such as additional system controls or a stand-alone de-humidifier.

Stand alone dehumidifiers can be both ducted and completely separate appliances. The major benefit of the stand alone dehumidifier is the ability to control the indoor relative humidity in the home without having a major effect on temperature. This is very valuable for climates that have a low sensible load but higher humidity (latent load) such as coastal climates.

A ducted dehumidifier can be integrated with the central air conditioning distribution system as shown in Figure 2 and Figure 3. The fan inside the dehumidifier unit draws air from the living space or from the air handler plenum and returns it to the supply plenum. From there, the dehumidified air is distributed throughout the home through the HVAC ducts. A remote dehumidistat located in the living space can trigger the dehumidifier to come on when indoor humidity levels exceed a set point. Controls connecting the air handler to the dehumidifier can activate the central air handler fan when the dehumidifier is activated. Controls could also be configured to operate a damper on a duct that brings outside air to the return side of the air handler to prevent overventilation with humid outside air during excessively humid periods.

Figure 2. A supplemental dehumidifier can be integrated with the central air handler to use the HVAC ducts for air distribution, with controls including a dehumidistat located in living space to switch on the dehumidifier and the central fan when extra dehumidification is needed.

Figure 3. A supplemental dehumidifier is integrated with the home’s HVAC air handler to provide extra dehumidification when needed (Image courtesy of BSC).

Additional Dehumidification Controls

With additional dehumidification controls on the HVAC system, the cooling system is operated with a slower fan speed to allow for the air to spend more time on the cold coil surface. This results in additional dehumidification. Because the system is still operating as an air conditioner, also dropping the temperature of the home, this may result in potential comfort issues.

Adding controls to the HVAC system is generally less expensive than installing a central or stand-alone dehumidifier. However, they are also more suited for climates with shorter shoulder seasons, because of the potential comfort issues while running the air conditioner in cooler weather. The stand-alone dehumidifier may be a more costly upfront investment, but this option generally costs less to operate and does so without the same risk of comfort issues in the home.

How to Conduct a Manual J Calculation

A contractor can utilize one of the HVAC-industry adopted software programs based upon Manual J to assist with calculating HVAC system design loads. Consult the ACCA website for a list of software programs to perform Manual J calculations.

A Manual J report includes:

Climatic data, such as the “Outdoor Design Conditions” (found in the ACCA Table 1 of MJ8). This information includes the local elevation, dry-bulb and wet bulb temperatures, as well as the daily temperature range.

Indoor design temperature and relative humidity for heating and cooling.

Heat loss and gain for heating and cooling for each of the following elements of building loads:

Windows and glass doors

Skylights

Wood and metal doors

Above and below grade walls

Ceilings/roofs

Floors/foundations

Air infiltration

Internal gains – appliances and occupants

Duct location and tightness

Ventilation loads

Note: Indoor airPLUS Specification 7.3 requires the builder to provide the homeowner with a signed copy of the HVAC, duct and ventilation system design documentation, including Manual J and Manual D analysis reports (Manual D requirements are discussed in Indoor airPLUS Specification 4.2). These reports should include room loads, duct system criteria by room and duct layouts (e.g., drawings illustrating return sizes, supply trunks, run-out duct sizes, and the cubic feet per minute (cfm) of conditioned air delivered to each room).

Ensuring Success

The HVAC contractor is required to conduct a Manual J calculation of HVAC system design loads and meet ENERGY STAR dehumidification requirements.

Climate

Warm-Humid climates include all of climate zone 1, all of climate zone 2A, and the portion of climate zone 3A below the white line as shown in the IECC climate zone map below.

IECC Climate Zone Map

Training

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Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

DOE Zero Energy Ready Home Notes

The U.S. Department of Energy (DOE) Zero Energy Ready Home National Program Requirements includes as a mandatory requirement (Exhibit 1, Item 6) that all homes meet the U.S. Environmental Protection Agency Indoor airPLUS Construction Specifications.

NOTE: Completion of the ENERGY STAR checklists now satisfies the following Indoor airPLUS requirements:

Properly size all heating and cooling equipment to accommodate design loads for each room as determined using ACCA Manual J, ASHRAE Handbooks, or equivalent software, as well as the pressure drop from all specified filters (HVAC-C Section 2).

A central HVAC system equipped with additional controls to operate in dehumidification mode.

Exception: Climate Zones 4-8, 3B, 3C and the portions of 3A and 2B above the white line as shown by 2009 IECC Figure 301.1.

Advisory: Although not required to meet this specification, independent dehumidification is recommended in Climate Zones 4A and 3A above the white line as shown in 2009 IECC Figure 301.1

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Standard covering sizing strategies for all types of cooling and heating equipment, as well as how to use comprehensive manufacturer’s performance data on sensible, latent, or heating capacity for various operating conditions.

Research study identifying the best performing, most energy-efficient and cost- effective techniques to provide controlled mechanical ventilation and humidity control in hot- humid climates with thermally efficient building envelopes.

This document covers a description of the need and applied solutions for supplemental dehumidification in warm-humid climates, especially for energy efficient homes where the sensible cooling load has been dramatically reduced.

Whole-House Dehumidification =

Whole-House Humidity Control

In hot, humid climates, reducing indoor humidity is an important part of making the homes’ indoor environment more comfortable, more durable, and healthier. In very humid climates, the air conditioner alone may not be able to remove enough latent heat to keep relative humidity below 60%. In such cases, supplemental dehumidification and humidity controls can be added to the HVAC system. This can include stand-alone dehumidifiers, a dehumidifier that is integrated into the central air conditioning system, or dehumidification controls that are added to the HVAC system to operate the cooling system for longer periods at a slower fan speed to allow more time for the coil to remove humidity from the air.

Alternate Terms

Advanced Humidity Control Technology

Whole-House Humidity ControlSales Message

Whole-house humidity control ensures comfort even when cooling is not required. What this means to you is that you will have a complete, not partial, comfort system. Wouldn't you agree humidity is too important not to manage at all times in new homes?

Last Updated: 03/11/2015

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